SAN CRISTÓBAL DE LAS CASAS, Mexico — Every morning, Sergio Castro crisscrosses this city to treat the intimate struggles behind its closed doors.

Past a black metal gate, Diego Raúl López Sánchez lay on a bed in a concrete room. A motorcycle crash left him paralyzed from the neck down a few months ago, and bedsores have branded his emaciated body.

Mr. Castro cleaned and dressed the broken skin as he murmured softly to his patient. He offered advice to Mr. López’s wife, who seemed numb with despair at her husband’s new reality. He would return the next day.

Neither doctor nor priest, Mr. Castro, 72, fills one of the countless holes in Mexico’s ragged safety net, which gapes wider here in the southern state of Chiapas than just about anywhere else in the country.

Over the past decade, Mexico has expanded access to medical services as successive governments have raised spending toward reaching the far-off goal of universal health coverage. But the sort of labor-intensive care that Mr. Castro provides is beyond the scope of rudimentary health centers in the countryside or the Hospital de las Culturas, a community hospital that opened here three years ago.

Don Sergio, as people here call him, spends much of his time patiently cleaning and bandaging wounds caused by burns or diabetes. He accepts no money from his patients, because “then they can be calm and they are more motivated to heal quickly,” he said.

“The ability, the gift that God gave me to do this — that is what gives me results,” he said.

When he can gather together enough from the donations that support him and his work — including from American expatriates living in Mexico — he helps villages build schools and treat their water.

Now his concern is the López family. Although Mr. López, 29, is covered by government insurance, there is no ambulance to take him for rehabilitation an hour away, and his disability claim is ensnared in the bureaucracy. Mr. Castro has been using donations to pay for physical therapy, but money is running out. He has been thinking ahead: planning for a tiny convenience store that Mr. López could run from home.

“This is a special case that I am committed to,” Mr. Castro said of his patient. “It’s indispensable for him to change psychologically, not to feel useless, to have some resources.”

Dressed like a retired rodeo hand in boots, faded black Levis and a Western hat, a packet of Salems tucked into his shirt pocket, Mr. Castro sees patients each afternoon at a small house that serves as both a clinic and a museum for his collection of indigenous clothing, masks and sculptures.

Many of his patients are Mayans from the surrounding highlands, who are among Mexico’s most marginalized citizens after suffering centuries of discrimination and neglect.

“He has worked himself into these different communities,” said Patricia Ferrer, a physician assistant in Tucson who visits Mr. Castro twice a year and sends him supplies. “He has developed a sense of trust, and that is why people seek him out.”

Víctor Jiménez Vicente, 29, arrived on crutches at the clinic recently, his foot engorged below an open wound on his leg from a decade-old burn that had never been treated properly. Speaking in Tzotzil, a Mayan language, Mr. Castro told him and his father that he suspected that the leg would have to be amputated and recommended that they go straight to the hospital.

But the thought of it stayed with him after they left. “I can’t just send them away and forget,” he said. “Instead of sleeping, my subconscious captures these problems.”

Poverty, mismanagement and negligence magnify the complexities of treating illness in Chiapas. Many people receive no preventive care at all.

When they do, “the capacity of services is very limited, and their quality is very deficient,” said Dr. Héctor Ochoa, the director of the health research group at the College of the Southern Border here.

Government programs to screen for cervical and uterine cancer, or to detect and treat tuberculosis and diabetes, fall short, he said. Test results go astray, patients get no follow-up care, medicine runs out.

“Unfortunately, in our political system health issues are pushed to second place,” Dr. Ochoa said.

At the Hospital de las Culturas, even the fire hose is labeled in Tzotzil and Tzeltal, the two main languages here — but there is not a single official translator to talk to patients’ relatives.

The hospital’s director, Dr. Marco Antonio Flores Pérez, 41, has more immediate priorities: how to attend the increasing number of patients who turn up in the emergency room, how to get the X-ray equipment fixed and how to ensure there are enough medical supplies — and make sure they are not stolen.

Many of the villages that the 60-bed hospital serves are very poor, and “part of the morbidity and mortality we see is because people use traditional medicine first,” Dr. Flores said. “At the end, they come to us with an infinity of complications.”

Dr. Hugo Cameras is in charge of a hospital annex intended to offer traditional medicine, but he has no budget for trained doctors or medicine. “We live in a racist, classist society,” he said.

The needs are so great that there is room for anybody who is committed to healing, Dr. Cameras said, including Mr. Castro, whom he has known for years.

“It’s in his treatment of people, that is how he cures,” Dr. Cameras said of Mr. Castro. “He helps a lot psychologically.”

Originally from northern Mexico, Mr. Castro arrived almost half a century ago to work as an agronomist and veterinarian in the highlands here. He quickly began treating the machete cuts and burns he saw as he traveled through villages.

Little has changed over the decades. Only a 20-minute drive up the mountain from San Cristóbal’s cobblestone streets, Carolina Hernández López, 34, a widow, cares for six children who all live with her in a one-room concrete block house with a dirt floor.

This year her 3-year-old daughter, Guadalupe, was scalded by boiling water that fell from a pot over the fire Ms. Hernández keeps in the middle of the room. Ms. Hernández took Guadalupe directly to Mr. Castro, who drove up the mountain every day after that for a month to treat the girl.

There are many other burned children. Amalia, 3, was brought to him after a month in the hospital did not heal her. Wilbur, 10, was scalded when he knocked over a pot of boiling water in his family’s cramped quarters.

Mr. Castro’s task has become much more complicated over the past few years in one important way. He now sees an increasing number of patients with uncontrolled diabetes, which has become one of the leading causes of death in Mexico.

At the crowded home of Cesar Morales, 59, who went blind six years ago from diabetes, Mr. Castro was greeted like an old friend. Crouching down, he dressed a diabetic ulcer festering on Mr. Morales’s leg.